Allow component declaration in VHDL blocks

[vmchale]

This allows us to handle component declarations in VHDL blocks.

It it necessary for VHDL support for IP as we use it, see #2182 (comment)

Still TODO:

• Check copyright notices are up to date in edited files

[martijnbastiaan]

LGTM from a purely functional POV. I've got a few engineering suggestions though.

Edit: Also usually I'd ask for a test, but I think we're implicitly going to do that for all cores in clash-cores in the near future :).

[DigitalBrains1]

I also noticed something I'd like to point out but I will do that tomorrow.

[martijnbastiaan]

LGTM, curious what @DigitalBrains1 has to say!

[vmchale]

I've rewritten with Alex's suggestions - I think this should be squashed when it is merged.